Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
It could evolve, strengthen, decay, or not be alone.
Our known Universe contains matter, radiation, and dark energy.
While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant. (E. SIEGEL / BEYOND THE GALAXY)
As it expands, matter and radiation dilute, but dark energy persists.
When we plot out all the different objects we’ve measured at large distances versus their redshifts, we find that the Universe cannot be made of matter-and-radiation only, but must include a form of dark energy: consistent with a cosmological constant, or an energy inherent to the fabric of space itself. (NED WRIGHT’S COSMOLOGY TUTORIAL)
As time progresses, only dark energy remains cosmically important.
Various components of and contributors to the Universe’s energy density, and when they might dominate. Note that radiation is dominant over matter for roughly the first 9,000 years, then matter dominates, and finally, a cosmological constant emerges. (The others do not exist in appreciable amounts.) However, dark energy may not be a pure cosmological constant. (E. SIEGEL / BEYOND THE GALAXY)
It determines our fate, but these five scenarios could irrevocably change it.
If dark energy were to decay from its current energy state to a lower-energy one, the fundamental constants would change and all matter in that transition region would become unstable, and would immediately be destroyed. A “bubble of destruction” would propagate outwards in all directions at the speed of light. We’d never see our own demise coming. (EU’S COMMUNICATE SCIENCE)
1.) Vacuum decay. Dark energy, the zero-point energy of empty space, possesses a positive, non-zero value.
A scalar field φ in a false vacuum. Note that the energy E is higher than that in the true vacuum or ground state, but there is a barrier preventing the field from classically rolling down to the true vacuum. Note also how the lowest-energy (true vacuum) state is allowed to have a finite, positive, non-zero value. The zero-point energy of many quantum systems is known to be greater than zero, which is what dark energy appears to be. We do not know if it’s a true or false vacuum. (WIKIMEDIA COMMONS USER STANNERED)
Decaying to a lower-energy state would create a destabilizing “bubble of destruction,” expanding outward at light-speed.
The two simplest classes of inflationary potentials, with chaotic inflation (L) and new inflation (R) shown. In both cases, rolling from high up on the potential down into the valley leads to the end of inflation and the start of the hot Big Bang. Conversely, a high-enough energy collision could restore the inflationary potential, and the inflationary state that preceded the Big Bang along with it. (E. SIEGEL / GOOGLE GRAPH)
2.) Restoring inflation. Cosmic inflation occurred very early on, preceding and setting up the hot Big Bang.
A hypothetical new accelerator, either a long linear one or one inhabiting a large tunnel beneath the Earth, could dwarf the sensitivity to new particles that prior and current colliders can achieve. If we ever reach collision energies comparable to the energy scales of inflation, it’s possible to restore the Universe to an inflationary state, destroying the vicinity around us in the process. (ILC COLLABORATION)
Particle collisions at sufficiently high energies — 10¹⁵ GeV or so — could restore the inflationary state, “resetting” our Universe.
The different ways dark energy could evolve into the future. If the future Universe sees dark energy increase in strength, we’re headed for a Big Rip scenario; if dark energy flips in sign, we could head for a Big Crunch instead. Although dark energy appears to be a constant today, alternative possibilities are not ruled out. (NASA/CXC/M.WEISS)
3.) Dynamical dark energy. Dark energy might not remain constant, but could evolve unexpectedly.
Recent constraints on the nature of dark energy are shown in the two graphs here. If dark energy is anything other than a cosmological constant, where w = -1, w_0 = -1, and w_a = 0, exactly, it will evolve with time. Our Universe’s fate will differ depending on the current values of these parameters, as well their time evolution. (PDG 2019; D.H. WEINBERG AND M. WHITE)
If it strengthens, weakens, or flips sign, a “heat death” may not await us at all.
The relative importance of dark matter, dark energy, normal matter, and neutrinos and radiation are illustrated here. While dark energy dominates today, it was negligible early on. If an even darker form of energy exists, it may not have become apparent yet, but could show itself to us and come to dominate the Universe in the distant future. (E. SIEGEL)
4.) There’s a “darker” energy out there. Dark energy’s constant density makes it important once matter and radiation dilute sufficiently.
If dark energy is a constant, it follows the blue solid/dashed line. However, a “darker” form of dark energy could also be present, starting out weaker and strengthening over time, eventually surpassing all other forms of energy, including the constant dark energy we appear to have. Eventually, it will become the only important component to the Universe. (QUANTUM STORIES)
A yet-undetected “darker” energy could strengthen over time, eventually dominating all other components in the Universe.
Just as a black hole consistently produces low-energy, thermal radiation in the form of Hawking radiation outside the event horizon, an accelerating Universe with dark energy (in the form of a cosmological constant) will consistently produce radiation in a completely analogous form: Unruh radiation due to a cosmological horizon. This, and other relations, could link large black holes with inflating/accelerating baby Universes. (ANDREW HAMILTON, JILA, UNIVERSITY OF COLORADO)
5.) Transport to another Universe. Black holes could serve as portals to other, baby Universes.
The exact solution for a black hole with both mass and angular momentum was found by Roy Kerr in 1963, and revealed, instead of a single event horizon with a point-like singularity, an inner and outer event horizon, as well as an inner and outer ergosphere, plus a ring-like singularity of substantial radius. An external observer cannot see anything beyond the outer event horizon. (MATT VISSER, ARXIV:0706.0622)
As our Universe’s final, persisting structures, black holes could provide a final escape.
Every supermassive black hole rotates, and has inner and outer ergospheres as well as an inner and outer event horizon. Some calculations suggest that before you ever reach the black hole’s singularity, you experience space as though it’s transported you to another Universe. This could serve as an escape from our dark energy-dominated Universe. (WIKIMEDIA COMMONS USER KJORDAND)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
